This invention is in the field of dial illumination means and is specifically directed to electrically operated dial illumination means usable with clocks, watches and other dial type instruments and the like such as meters, compasses, radios and a variety of other similar devices.
A number of expedients have been employed in the past for the purpose of providing illumination of watch dials. For example, phosphorescent materials have been employed for many years on watch dials with varying degrees of success and all suffering from the well-known shortcoming of being difficult to see particularly with the passage of time as the phosphorescence decreases. Moreover, the phosphorescent material requires recharging by being exposed to light in order for the phosphorescence to be maintained. In addition, the luminous materials used in dials of the foregoing type also lose their light emitting capacity with age and have generally provided substantially less than fully satisfactory performance. The shortcomings of the luminous markings have resulted in other approaches such as in the use of gas or incandescent bulbs in the manner of U.S. Pat. No. 3,018,614. Devices of the last-mentioned type employing incandescent filament operated bulbs have not been accepted for portable devices due to the fact that they require substantial battery capacity for their operation due to their substantial power requirements. Other prior art approaches to the provision of dial illuminating means are found in U.S. Pat. Nos. 3,214,577; 3,224,184; 3,278,740; 3,430,433; 3,490,266; 3,514,940; 3,574,993; 3,859,782; 3,748,456; 3,788,061; 3,855,784 and 3,899,871.
Probably the greatest shortcoming of the prior art devices is their reliance upon incandescent type bulbs employing filaments which have consequent high power requirements requiring substantial battery capacity for operation.
Another shortcoming of many of the prior art devices such as exemplified in U.S. Pat. No. 3,788,061 is due to the fact that they inefficiently rely upon a relatively powerful source which attempts to illuminate the entire area of the dial by simple radiation from the source so that those portions of the dial nearest the light source are usually overly illuminated in order that the portions spaced from the light source may have adequate illumination. Additionally, this approach tends to cause shadows especially with curved surfaces and three-dimensional objects. Other drawbacks of the prior known devices for providing dial illumination include the fact that they are bulky and are frequently both delicate and expensive.